This invention relates to a thermal radiation baffle for apparatus for use in the directional solidification of crystalline material, in particular, to obtain high strength articles of metal alloys.
In one general form for such apparatus a furnace, in which molten material is poured into a mould to cast the required article, is mounted vertically above means for rapidly cooling the molten material in the mould. The means for rapidly cooling the molten material may have one of several different constructions. In general, such means comprises a chill plate on which the mould is mounted, and the arrangement associated with the plate is so that heat can be extracted therefrom, and hence also from the mould, at a high rate. The chill plate is movable, in a controlled manner in a vertical direction, and drive means for the plate is provided. The drive means initially causes the mould to be located in the furnace; and then causes the mould to be withdrawn from the furnace, so that the molten material then in the mould is solidified.
It is required for the production of high strength articles by directional solidification that the molten material in the mould should start to solidify at the chill plate. Further, the arrangement is to be so that a sharply defined interface between the solidified and molten material is obtained; and that this interface moves in the required direction of crystallisation of the material. For the general form of apparatus referred to above, this direction of interface movement is opposite to the vertical direction of relative movement of the chill plate and filled mould away from the furnace. The speed of movement of the chill plate can have any desired value; but it is required that heat is removed from the chill plate as quickly as possible; and that the rate of heat supplied to the parts of the mould displaced from the furnace by radiation from the furnace, is as small as possible. In order that the latter criterion is obtained it is known to provide a thermal radiation baffle of a refractory material mounted at the lower end of the furnace, and through which baffle the filled mould is withdrawn. The empty mould is also required to pass through the baffle in the reverse direction.
Many different constructions for such a thermal radiation baffle are known. For example, it is disclosed in U.S. Pat. No. 3,714,977 to provide a layer of shell mould material; with an overlying layer of graphite fibres, this fibre layer having a random construction to comprise a felt; and there being a permanent aperture for the passage of the mould through the centre of the baffle. The two layers are secured together, with the inner periphery of the apertured felt layer extending beyond the corresponding inner periphery of the other layer, substantially into contact with the mould. The felt is flexible to a degree, and the mould may move freely past the edge of the felt layer, this edge being capable of deforming to some extent if necessary, and if contacted by the mould.
It is an object of the present invention to provide a novel and advantageous construction for a thermal radiation baffle for apparatus for use in directional solidification.